These years, much attention has been focused on thin-film EL device using a thin-film phoshor layer instead of a dispersion EL device using zinc sulfide (ZnS) compound phoshor powder, because the former can provide a high luminance.
The thin-film EL device, in which a luminous layer is made in the form of a thin film to minimize halation or luminous blur caused by the scattering of externally incident light and of light emitted from the interior of the luminous layer and to thereby offer high sharpness and contrast, has been put on the stage as a device for mounting on vehicles, as such a display unit as in a computer terminal, or as an illumination device.
In the case where the thin-film EL device is used as a display, the EL device is constructed as a dot matrix type shown in FIGS. 2(a), (b) and (c) so that light-permeable surface and back electrodes 101 and 102 each takes the form of a pattern of stripes arranged as mutually spaced at intervals of a predetermined distance and as mutually intersected at a right angle, a luminous layer 103 is interposed between the surface and back electrodes, each of intersections between the surface and back electrode patterns forming one of the elements of the thin-film EL device. FIG. 2(a) is a perspective view, partly as cut away, of the EL device, FIG. 2(b) is a plan view thereof and FIG. 2(c) is a cross-sectional view thereof.
More in detail, first and second insulating layers 104 and 105 are disposed between the luminous layer 103 and the surface and back electrodes 101, 102 respectively.
And the light emitting process of this thin-film EL device is as follows.
First, when a voltage is applied between the patterns of the surface and back electrodes 101 and 102 in response to an input signal, electric fields are induced in the luminous layer 103 at the intersections between these patterns so that electrons so far trapped at the interface level are released therefrom and accelerated, whereby the electrons acquire sufficient energy to be bombard with orbital electrons of luminous center impurities, thereby exciting the orbital electrons. When the thus excited luminous center electrons return to the ground or normal state, they emit light.
With such a thin-film EL device, the electrodes of the elements of the EL device are provided at their one ends with terminals 106 which are to be connected to an external controller (not shown) through associated lead wires and which terminals are usually made of a nickel (Ni) film.
Such terminals 106 have been conventionally formed by an electron beam evaporation process after the sequential formation of the surface electrode 101 pattern, first insulating layer 104, luminous layer 103, second insulating layer 105 and then back electrode 102 pattern.
In the patterning process, the nickel thin-film pattern has been made usually by a selective evaporation process, that is, by the electron beam evaporation process using a metal mask.
This selective evaporation process, however, has had such a problem that the pattern edge does not correspond exactly to the mask, that is, the pattern shape cannot be sharply defined with a bad pattern accuracy.
Such an EL device has also been defective in that, because the electrodes of the device have a high pitch of about 0.5 mm, it is highly difficult to align such a fine pattern with the metal mask and thus its yield is reduced due to the positional shift in the pattern.
To eliminate such problems, there has been proposed a method in which terminal patterning is carried out by a photolithographic process.
In this method, as shown in FIG. 3(a), elements are first formed and then a nickel thin film 106' are made in the form of a strip by the electron beam evaporation process.
Subsequently, patterning is effected by the photolithographic process to form a nickel terminal 106, as shown in FIG. 3(b).
This method is advantageous in that the pattern accuracy is improved but disadvantageous in that the impurity ions or moisture often causes the deterioration of the elements of the EL device in the etching step and further the number of steps in the photolithographic process is large, resulting in that the cost of the associated photo masks is high, and so on.
It is generally known that the elements of a thin-film EL device are subjected to a damage by moisture or impurity ions in the etching step. This phenomenon has been a serious problem in the thin-film EL device, since the device is operated, in particular, under a high electric field so that the frequent use of the device causes the moisture adsorbed on the device in the electric field to be broken down and penetrated into the interfaces of the films, thus causing the film release and involving the shortened operational life.
In view of the above circumstances, it is an object of the present invention to provide a thin-film EL device which is easy to fabricate and high in reliability.